Acidizing is a process for dissolving material from a formation to improve production. Acidizing fluids have been used to increase the productivity of oil and gas from calcareous formations by effecting the removal of reactive materials from naturally occurring fractures and pore spaces in the formations whereby the sizes thereof are increased. Acidizing fluids also have been used to create new fractures in formations with the acid acting to etch the fractures so that they remain open and have a high flow capacity.
An acidizing fluid can be introduced into the formation only a certain distance before it becomes spent. In creating new fractures in a formation, if the acidizing fluid is pumped under pressure further into the formation after it has become spent, it may extend fractures in the formation, but it may not increase the flow capacities (e.g., via acid-etching) of the extended fractures. Strong mineral acid such as 15% hydrochloric acid can be used to fracture limestone reservoirs with a view to improving production from the rock. The technique generally involves pumping the acid at high pressure into the rock so as to induce a fracture in the rock, and, generally speaking, this fracture will create a dendritic pathway deep into the interior of the rock. The acid dissolves carbonate rock from the walls of the fracture thus etching a pathway through which the produced oil or gas can flow back to the production string. However, the acid spends at the surface of the carbonate rock, and it is thus very difficult for live acid to affect the dimensions of the pore throats that are naturally present in the rock. In other words, the permeability of the limestone in the near-wellbore/near-fracture zone will not be affected by the action of the live acid.
Compounds that can behave as a delayed release source of acid are not effective as the principal active agents in acid fracturing. Generally, the acids released are weak organic acids, and the rate of release of acid, depending on temperature, can be fairly slow. Thus, acid-etching of fractures is very difficult to achieve simply by relying on a delayed release acid.